The “Counterfeit Bill” Math Problem

photo (1)According to George Polya, famous mathematician, “A problem is not a problem if you can solve it in 24 hours.”  This month, over 200 PUSD 5th graders, their teachers, and parents were given a math problem that they continued to debate, long after 24 hours had passed.  Give it a try and see what answer you get.

A customer enters a store and purchases a pair of slippers for $5, paying for the purchase with a $20 bill.  The merchant, unable to make change, asks the grocer next door to change the bill.  The merchant then gives the customer the slippers and $15 change.  After the customer leaves, the grocer discovers that the $20 bill is counterfeit and demands that the shoe-store owner make good for it.  The shoe-store owner does so, and by law is obligated to turn the counterfeit bill over to the FBI.  How much does the shoe-store owner lose in this transaction?

If you’d like to know if your solution is correct, find a 5th grader and discuss your thinking!

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